The present invention relates to an apparatus and a method for determining a failure of a temperature sensor for an automatic transmission.
FIG. 3 shows a known apparatus for determining a failure of a temperature sensor for an automatic transmission of a motor vehicle. The apparatus includes an engine 1, an automatic transmission 2, a first temperature sensor 3 in the form of a coolant temperature sensor for sensing the temperature of a coolant for cooling the engine 1, an air flow sensor 4 for sensing the amount of intake air sucked into the engine 1, a rpm sensor 5 for sensing the number of revolutions per minute of the engine 1, a throttle sensor 6 for sensing the degree of opening of a throttle valve (not shown), an injector 7 for injecting a metered amount of fuel to the engine 1, an electronic control unit (ECU) 8 for controlling the injector 7, a second temperature sensor 9 in the form of an oil temperature sensor for sensing the temperature of hydraulic oil in the transmission 2, a speed sensor 10 for sensing the speed of a vehicle based on the rotational speed of a rotation shaft (not shown) in the transmission 2, a solenoid 11 for performing gear change or lockup of the transmission 2, and an electronic automatic transmission control unit (EAT) 12 for controlling the solenoid 11. The electronic control unit 8 and the electronic automatic transmission control unit 12 are connected with each other through a signal line or they are accommodated in a single unit. The electronic control unit 8 controls the injector 7 based on the outputs of the first temperature sensor 3, the air flow sensor 4 and the rpm sensor 5. The electronic automatic transmission control unit 12 controls the speed-change and lockup solenoid 11 based on the outputs of the throttle sensor 6, the second temperature sensor 9 and the speed sensor 10 so as to perform speed-change control.
FIG. 4 illustrates an interface circuit of the electronic automatic transmission control unit 12 for the second temperature sensor 9. The output of the second temperature sensor 9 is divided by resistors R1, R2 which are series connected with each other between a power supply of 5 volts and ground, then subjected to analog to digital conversion by an A/D converter 13, and fed to a central control unit (CPU) 14.
With the above-described apparatus, a failure of the second temperature sensor 9 is determined by detecting whether or not the output of the second temperature sensor 9, which is converted from analog to digital form by the A/D converter 13, is outside a predetermined range. Specifically, there exists a certain relationship between the temperature of hydraulic oil in the transmission 2 and the A/D converted output of the second temperature sensor 9, as shown in FIG. 5. When the second temperature sensor 9 is open, the output of the A/D converter 13 is nearly of 5 volts, whereas it is approximately zero when the second temperature sensor 9 is short-circuited. Accordingly, the CPU 14 performs failure determination using a first failure determination level which is slightly higher than the output of the A/D converter 13 when the sensor 9 is short-circuited, and a second failure determination level which is slightly lower than the A/D converter output when the sensor 9 is open. As can be seen from FIG. 5, however, the A/D converted output of the second temperature sensor 9 becomes close to the first and second failure determination levels at high and low temperatures of the hydraulic oil. As a result, it could be possible to determine, due to variations in the operational characteristic of the sensor 9 itself or variations in the operating characteristic of the interface circuit of the electronic automatic transmission control unit 12, a failure of the sensor 9 in spite of the fact that it is normal.